In the metabolic pathway of Klebsiella pneumoniae, 2,3-butanediol is a byproduct and its accumulation decreases the yield of main product, D-lactic acid. Butanediol dehydrogenase (BUD) is one of the key enzymes for butanediol biosynthesis, which competes with lactate dehydrogenase (LDH) for reducing equivalents of nicotinamide adenine dinucleotide (NADH). In order to reduce the butanediol accumulation and improve the D-lactic acid production, in this study, a homologous recombination vector pTBT was constructed and transformed into K. pneumoniae, resulting in K. pneumoniae B-, in which the budC gene encoding butanediol dehydrogenase was disrupted by inserting a tetracycline resistant gene (tet). Simultaneously, the expression vector pKP-ldhA harboring the ldhA gene was constructed and transformed into K. pneumoniae B- to overexpress lactate dehydrogenase. The resulting recombinant strain K. pneumoniae B-L+ exhibited a nearly abolished butanediol formation (decreased by 90%) but a significantly improved NADH availability and D-lactic acid production. In flask culture, the D-lactic acid concentrations were 77.1% and 41.4%, respectively, higher than those of the parent strains, K. pneumoniae B- and K. pneumoniae. In fed-batch fermentation with glycerol, the D-lactic acid yield, conversion and productivity reached 68.4g/L, 0.78, and 1.22g/(L·h), respectively.
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Footnotes
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